1. Field of the Invention
The present invention relates to a technology for controlling power supply, and more particularly for suppressing a surge current in an image forming apparatus having an interlock switch.
2. Description of the Related Art
Safety standards demand that an interlock circuit be provided in image forming apparatuses to prevent harm on a human body. For example, when an operator opens the front door of an image forming apparatus, motors or generators in the image forming apparatus are turned off. As a result, the operator can safely touch, handle, or repair the internal parts of the image forming apparatus. For example, when the operator opens the front door to take care of a paper jam while the power is on, control is provided so that the load driving voltage is not applied to the motors. The temperature and voltage are also prevented from increasing in this state, thereby avoiding an unexpected accident.
To realize this configuration, a power supply line to be opened or closed as an interlock circuit is wired to the front cover of the machine, and the power supply line is turned on and off with a switch for turning on and off the circuit based on a front-cover opening and closing mechanism. However, according to this method, when the load to be interrupted as the interlock circuit is a large current, the wire through which this large current flows needs to be arranged up to the front cover. This is not desirable from the viewpoint of suppressing electromagnetic radiation noise. Because of the large current, the harness of the power supply line needs to be thick. This thick harness is usually wired from the DC power supply device, which is fitted to a back lower part of the machine, to the front cover of the machine. An extra cost is required for this thick and long harness.
When the current that flows through the interlock circuit is 16 amperes, for example, at least three switches are necessary assuming that a rated current of a switch generally used is 6 amperes (6 (amperes)×3=18 (amperes)). However, when the current of 16 amperes flows at the rated current 18 amperes of the switch contact, there is little margin, and distribution of the load becomes difficult. Therefore, four switches may be necessary instead of three switches, taking margin into account. When the front cover is fitted to the left and the right sides of the machine, the number of switches fitted to the front covers becomes eight (4×2=8). However, in reality, a switch exclusively used for a laser power supply line, and a cover open/close detection switch are also necessary. Consequently, 12 switches are necessary in total (8+2(for laser)+2(for open/close detection)=12).
However, an increase in the number of switches increases the cost. In addition, space for installing six switches is necessary at one side of the front cover opening and closing mechanism. The six switches need to be turned on and off by securely linking the switches to the opening and closing of the front cover. Thus, the configuration of the opening and closing mechanism becomes complex, and design of the configuration becomes difficult.
To cope with the above problems, instead of using switches in the opening and closing mechanism, there is also a method of using a relay to interrupt the power supply voltage line of the load system, and interrupting the relay driving circuit with the interlock switch (the front cover switch). In this case, a current up to 20 amperes can be passed using two relays, when one relay has one contact of a rated current 10 amperes. In the case of a machine in which a current of 16 amperes flows, two relays have a function equivalent to that of eight switches. Therefore, an interlock circuit can be prepared using two relays and six switches (4(for laser)+2(for open/close detection)=6).
As a first conventional technique, Japanese Patent Application Laid-open No. H09-212042 discloses a method of suppressing a flowing current so as not to exceed a constant level when the cover is closed, to prevent the occurrence of an inrush current in an input smoothing unit of a drive load unit due to the opening and closing of the cover, thereby preventing a malfunction, and a technique of turning off the converter when the cover is opened and turning on the converter when the cover is closed.
As a second conventional technique, to cancel a timing deviation of contact on/off between interlock switches and prevent malfunction of a machine due to the use of plural interlock switches, Japanese Patent Application Laid-open No. 2002-345148 discloses a technique of using both an interlock switch and a relay, thereby determining an on/off sequence of each power supply system, canceling a timing deviation, and directly stopping generation of a voltage when the interlock switch is off. According to the second conventional technique, the occurrence of an inrush current when an off state of the interlock switch is changed to the on state can be prevented.
However, when the relay contact is closed, an inrush current flows due to a capacitive load provided at the load side, and this inrush current generates an arc and fuses the contact. When the relay contact is fused, the interlock circuit does not function.
To cope with this problem, one approach is to detect an abnormal operation of the interlock circuit before a fusing occurs, thereby preventing the fusing. However, the problem generated by the inrush current is not limited to the fusing of the relay contact. When the inrush current flows to the capacitive load, an overcurrent detecting circuit of a DC power supply circuit can malfunction. When the inrush current occurs, the voltage of the power supply voltage line drops. This causes malfunction of a central processing unit (CPU).
According to the first conventional technique, an inrush current is suppressed using a current limiting unit that limits a current flow to the drive load unit, and a bypassing unit that bypasses the current limiting unit. According to this technique, the circuit becomes complex, and the control of the current limiting unit becomes difficult.
Furthermore, according to the second conventional technique, the interlock is locked and unlocked to match the timings when plural power supply systems are turned on and off. A delay circuit and others are necessary for this purpose. Therefore, the circuit becomes complex, and the cost increases.